Heretofore known internal combustion engine misfire detection apparatus, detect cylinder pressure with a cylinder pressure sensor and integrate the detected value over a predetermined interval to obtain the indicated mean effective pressure. This is then compared with a predetermined misfire level, and when equal to or below the predetermined misfire level, misfire is judged and a warning generated.
With such misfire detection apparatus however, since the misfire level is fixed, changes with time (due for example to sensor deterioration or a drop in compression ratio due to cylinder leakage and the like) cannot be allowed for, so that the occurrence of erroneous output becomes a problem, particularly in ranges such as at low load where the noise/signal ratio level is high.
In view of the above problem with the conventional technology, it is an object of the present invention to improve detection accuracy by allowing for changes with time.